SUMMARYOptimum conditions for the in vivo nitrate reductase assay in soybean {Glycine max (L.) Merr.) foliage were determined. The in vivo and in vitro nitrate reductase assays correlated highly but exhibited differential decay rates with increasing leaf age. Nitrate reduction could be coupled to the oxidation of either frustose-i,6-diphosphate or 6-phosphogluconate. Compounds, such as glucose, 2,4-dinitrophenol or phosphate, which stimulate glycolysis also stimulated nitrate reduction. Citrate stimulated nitrate reduction in young expanding leaves but was inhibitory in mature, expanded leaves. Caffeic acid was inhibitory in young but not in mature leaves.